(1) Field of the Invention
The present invention lies in the field of powering rotary wing aircraft having a plurality of engines, and it lies more particularly in the field of regulating such a power plant.
The present invention relates to a power plant for a rotary wing aircraft and to a rotary wing aircraft having such a power plant, and it also relates to a method of regulating such a power plant. The invention is particularly intended for regulating a power plant having three engines.
(2) Description of Related Art
A power plant for a rotary wing aircraft generally has one or two engines and a main gearbox (MGB). Each engine drives the MGB mechanically in order to rotate at least one main outlet shaft of the MGB. The outlet shaft is constrained to rotate with a main rotor of the rotary wing aircraft in order to provide the aircraft with lift, and possibly also with propulsion.
The MGB generally has secondary outlet shafts, e.g. for driving rotation of a tail rotor or indeed of one or two propulsion propellers via an auxiliary gearbox and for driving an electricity generator and/or hydraulic systems. The respective frequencies of rotation of such secondary outlet shafts are generally different from the frequency of rotation of the main outlet shaft.
It should be understood that the term “engine” is used to designate an engine unit serving to drive said MGB mechanically, and consequently contributing to providing the rotary wing aircraft with lift and/or propulsion. Such engines may for example be turboshaft engines on board rotary wing aircraft.
Furthermore, it is nowadays common practice to use a two-engined power plant on board rotary wing aircraft, each engine being controlled by a dedicated computer. Such engines are generally identical turboshaft engines operating in application of regulation rules.
For example, regulation may be proportional, thereby enabling the system to be regulated in proportion to a difference between a current value of the system that is to be regulated and a setpoint value. Such regulation is generally effective. However the setpoint value is never reached when using proportional regulation, since there is always a difference between the current value and the setpoint. It is indeed possible to approach the setpoint by reducing the difference, however the system then often becomes unstable.
In order to improve such regulation, it is possible to introduce an additional correction for eliminating errors in tracking the setpoint. This correction is proportional to the time integral of the difference between the current value and the setpoint, i.e. proportional to the sum of all of the differences as measured continuously. This is known as proportional integral (PI) regulation.
Proportional integral derivative (PID) regulation also exists and includes an additional correction proportional to the derivative of the difference. This correction serves also to take account of variations in the difference, whether they are in direction and/or in amplitude.
PI regulation is frequently used on twin-engined aircraft, thus providing good control over the frequency of rotation of the main rotor and also over the performance of the aircraft. Operation is then balanced between the two engines of the power plant, thus making it possible in particular to have symmetrical wear of those engines and also in the mechanical connections constituting inlets to the MGB.
However, PI regulation requires complex connections between the computers of the two engines in order to ensure that each of the engines delivers equivalent power. In particular, such PI regulation requires the use of a balancing loop between the two computers.
In addition, the computers need to be relatively high performance computers in order to achieve such regulation. By way of example, such computers may be full authority digital engine control (FADEC) computers. The computers are often also twin-channel computer, i.e. the connections between the computers and also the connections between the computers and the engines are duplicated in order to make those connections secure, and consequently in order to make the operation of the power plant secure.
In addition, since the size of rotary wing aircraft is tending to increase, the need for power from their power plant is also tending to increase. Consequently, the power plants of such aircraft need to have at least three engines in order to be capable of delivering enough power.
Three-engined rotary wing aircraft are nowadays usually fitted with three engines that are identical, thereby making it possible in particular to ensure that the power plant is reactive in the event of one engine failing and also in order to simplify installing and integrating the engines.
Engines are said to be “identical” when they have identical characteristics for driving a rotary member.
Conversely, engines are said to be “unequal” when they have distinct drive characteristics, namely engines that generate different maximum powers and/or unequal maximum torques, and/or different maximum rotation frequencies of an outlet shaft. Thus, two unequal engines can correspond respectively to one engine driving an outlet shaft at several tens of thousands of revolutions per minute (rpm) and to another engine driving an outlet shaft at less than 10,000 rpm, for example.
For a power plant having three identical engines, the regulation of all three identical engines is identical, each engine delivering an equivalent level of power.
The use of such PI regulation with such a power plant nevertheless raises several problems. Firstly, it is necessary to use high performance computers such as computers of the FADEC type together with connections between each of the computers and each engine. The architecture of the connections between the computer then becomes complex, and the same applies to the regulation loop. This architecture is necessary in order to regulate the frequency of rotation of the main rotor in a manner that is simultaneously reactive, stable, and without static error. As a result, the balancing loop between those computers becomes complex and long.
Furthermore, since the computers may be twin-channel computers, the complexity of the connections between the computers becomes much greater. The weight of the electrical harnesses providing these connections can then be significant and installing them in the aircraft can also become complex. Consequently, the cost of such regulation for the at least three engines in such a power plant can become very high.
It is also possible to use proportional regulation for each engine. The architecture of the connections between the computers is then simpler, while still enabling the engines to be balanced. Nevertheless, static errors in the frequency of rotation of the main rotor of the aircraft are then frequent and the performance of the power plant and consequently of the aircraft is not fully under control and therefore not optimized.
It is also possible to use unequal engines in a three-engined power plant, e.g. in order to comply with safety requirements or in order to mitigate a lack of power from engines available on the market.
However, for a three-engined power plant having at least two unequal engines, regulating the engines can become even more complex, in particular in terms of sharing the power from each engine and regulating the frequency of rotation of the main rotor.
The technological background includes Document U.S. Pat. No. 4,479,619, which proposes a power transmission system for three-engined helicopters. Likewise, the Applicants' Super-Frelon helicopter also has three identical engines.
Document U.S. Pat. No. 3,963,372 proposes a solution for managing power and controlling engines in three-engined helicopters.
Document US 2003/0135305 describes a system for anticipating the torque drawn by the main rotor of a rotary wing aircraft, in particular in order to avoid a drop in the frequency of rotation of the main rotor.
Also known is Document U.S. Pat. No. 3,174,551, which describes a device capable of controlling and correcting unbalance between the powers delivered by two turboshaft engines of an aircraft.
Furthermore, Document U.S. Pat. No. 4,522,025 describes a system for managing the power and the frequency of rotation of the turboshaft engines of a rotary wing aircraft.
In order to mitigate the problem of engines that are designed so as to be overdimensioned, a power plant having engines with unequal maximum powers, for twin-engined aircraft, has already been proposed in the past. This applies to Document WO 2012/059671A2, which proposes two engines having unequal maximum powers. Nevertheless, that Document WO 2012/059671A2 relates only to twin-engined aircraft and does not present any solutions to problems associated with control or stability.